You have to use the {alias|...} code to expand aliases in MS-DOS buttons, since MS-DOS doesn't understand Opus aliases.
(In fact, this may be true for arguments to external commands in general, but I'd have to check to be sure.)
If aliases were expanded automatically in this situation, having a /S alias would make it impossible to run a DOS command like dir /S where the /S is intended to be an argument switch, not an alias.